1. Field of the Invention
One or more embodiments of this invention relate to an engine emission control system having a coated mixer and its method of use.
2. Background Art
Environmental regulations pertaining to the reduction of emissions from vehicular engines have been active in many countries. The oxides of nitrogen are of concern regarding emissions and include several compounds such as nitric oxide and nitrogen dioxide. These compounds are frequently referred to as NOx as defined by the United States Environmental Protection Agency.
NOx emissions from certain engines may be treated using a selective catalytic reduction (SCR) catalyst. The SCR system uses reducing agents, such as urea to yield satisfactory NOx conversion performance. Some recently designed delivery systems for the reducing agent involve delivery of a gas to the SCR.
In some systems the gas is generated by vaporizing a liquid reducing agent. Under these types of systems, vaporization of the liquid reducing agent to a gas can sometimes be incomplete. As a result, urea droplets may reach the face of an SCR catalyst. These droplets could lead to deposits of melamine and other solids resulting in diminished SCR performance. In addition, when the droplets or other materials are not evaporated and decomposed prior to contact with the SCR catalyst, part of the SCR catalyst must be used for this decomposition by hydrolysis. As a result, the SCR catalyst may be required to be relatively larger to achieve the same level of NOx conversion than if the droplets were not present.
What is needed is a system for thoroughly vaporizing and mixing the exhaust and the liquid reducing agent. The system should provide good effect during cold start engine NOx conversion operations while avoiding detrimental increases in back pressure on the emission control system.